There is a holographic recording and reproducing method and apparatus available as one of high-speed mass storage technologies that allow for storing massive amounts of digital information.
This holographic recording and reproducing method and apparatus stores digital information as data pages formed of a two-dimensional bit map image with each page being multiplexed.
Such a holographic recording and reproducing method and apparatus provides not only a high-speed massive storage capability but also a retrieval and reproduction capability referred to as the “associative reproduction” or a very-high-speed retrieval capability of instantaneously finding out specific data using the volumetric nature of holograms.
An example of this is described in APPLIED OPTICS, Vol. 38, No. 32, p 6779 to 6784, 10 Nov. 1999. In this example, a holographic recording medium having data pages recorded thereon with each page being multiplexed is irradiated with a signal beam that has been modulated by a spatial light modulator using a data image being retrieved, thereby allowing diffracted beams to be transmitted. Of the resulting diffracted beams, the diffracted beam of the maximum intensity is used to detect the data page of the data image provided to the signal beam and the address of a reference beam associated therewith.
Here, the hologram retrieval method according to the aforementioned holographic recording and reproducing method and apparatus allows for detecting a diffracted beam on one data page basis. Thus, the method has a problem that retrieval of small size data causes the amount of diffracted light to be decreased, resulting in the accuracy of retrieval (SNR) being degraded.
In contrast to this, an increase in the exposure time of a signal beam for retrieval causes an increase in noise light, and thus the SNR cannot be prevented from being reduced.
There is also another problem that the retrieval accuracy is reduced or the retrieval itself is made impossible as the size of data used for retrieval is reduced.
This is caused by increases in crosstalk between pages at the time of retrieval depending on the similarity of data images associated with respective data pages (or depending on the number of common ON pixels).
That is, a decrease in size of an image zone used for retrieval makes it difficult to ignore the similarity in the other zones (or accidental matching). Thus, this raises a problem of increasing the probability of “retrieval error,” in the case of which with no correlation being found in searched pixel zones, a strongly correlated data page is extracted in other zones as a result of a search.